This invention relates to bladed aircraft rotors, for example bladed helicopter sustaining rotors, and while various aspects of the invention are adapted to and useable in other bladed rotors, such as a so-called tail rotor of a helicopter for controlling the aircraft in yaw, the invention is herein described and illustrated as embodied in a helicopter sustaining rotor.
In conventional aircraft sustaining rotor systems, particularly power-driven helicopter rotor systems, it has been customary to employ rotor blade mounting pivots, usually including at least two pivots for each blade, i.e., a blade flapping pivot, and a pitch change pivot. So-called drag pivots are also quite commonly employed. The blade pivots have been utilized extensively for the purpose of providing freedom for the required blade motions, such as the flapping motion, the pitch change motion, and the lead-lag motions of the blades, those various motions being desirable in various flight operations, particularly in controlled maneuvering and translational flight. While the blade pivot structures referred to have served those purposes, the employment of such pivots involves expensive manufacturing costs and requires costly maintenance, and in addition, represents potential safety hazards in the operation of such aircraft.
In order to eliminate the disadvantages incident to the employment of blade mounting pivots, certain flexible blade mounting arrangements have been proposed, as disclosed, for example, in my prior U.S. Pat. No. 4,266,912, issued May 12, 1981, and U.S. Pat. No. 4,502,840, issued March 5, 1985, the disclosures of which are incorporated herein by reference.
In the '912 patent, there are disclosed flexible straps (herein referred to as flex straps) employed for connecting the root ends of the rotor blades to a hub structure, the straps providing freedom for various blade motions with respect to the hub structure, including motions in the flapping plane, lead-lag motions, and pitch change motions. In the '840 patent, flex straps for mounting the blades are also disclosed, together with controllable pitch change mechanism.
The present invention contemplates employment of flex strap arrangements and pitch control mechanisms of the kind disclosed in my prior patents above-identified, but in addition, the present application is concerned with a number of improvements in rotors of this type in which blade mounting pivots are not used.
In considering one aspect of the present invention, it is first noted that the required flapping motions of the rotor blades represents substantial motions in the flapping plane, such flapping motions originating from various sources, including differential lift compensation at opposite sides of the rotor in translational flight, and also extensive motions in the flapping plane resulting from imposition of the cyclic pitch control, with resultant change in attitude of the aircraft, which frequently requires quite large additional flapping motions superimposed upon those resulting from differential lift compensation in translational flight.
With the foregoing in mind, the present invention contemplates the employment not only of the flex straps above referred to, but also of a flexible joint at the center of the rotor or rotor hub mechanism, which flexible joint will accommodate conjoint equal and opposite flapping motions of the blades such as are frequently encountered in maneuvering control of the aircraft. By the employment of this central flexible joint, the extent of flapping motions required to be accommodated by the flex straps is greatly reduced, so that, in effect, the overall flapping motions are divided and distributed between two different mechanisms, instead of relying solely upon the flex straps for all of the flapping motion accommodation.
For the above purpose, the invention contemplates employment of a spherical center joint through which the blades are connected with a central mounting structure, for instance, the rotor drive shaft, and as will be explained more fully hereinafter, the form of the central flexible joint employed not only accommodates and absorbs a portion of the flapping motions of the blades, but also is resistant to deflection in a direction parallel to the axis of rotation, and, therefore, serves as the load transmitting component between the body of the aircraft and the sustaining blades
The foregoing accommodation of different portions of the flapping motions by different mechanisms is of importance in connection with the utilization of the flex straps, because the reduction in the overall flexibility requirements of the flex straps makes possible the employment of a wider range of materials in the fabrication of the straps themselves.
In addition, the employment of the central spherical flexible joint accommodating a portion of the flapping motions of the blades results in the concentration and handling of the equal and opposite flapping motions of the blades at opposite sides of the rotor about the center point of the central joint. Since this center point lies on the axis of rotation of the rotor, this diminishes the Coriolis effect which would otherwise be set up as a result of the total flapping action being absorbed by the flex straps, as will be explained more fully hereinafter.
In accordance with another aspect of the present invention, a novel form of torque transmission mechanism is employed between the drive shaft for the rotor and the rotative parts of the hub structure, this drive transmission comprising flexible sheet material in the form of bellows mounted above and below the plane of connection of the rotor blades to the central flexible joint. Edges of the bellows are respectively connected with the drive shaft and the blade mounting parts of the rotor, thereby providing a constant torque drive system, even when the blades occupy equal and opposite flapping positions at opposite sides of the rotor. By the employment of bellows both above and below the plane of the blade mounting, the flapping motions of the blades provided by the central joint are not only accommodated, but in addition, the torque loads are divided between the upper and lower bellows, thereby making possible the employment of economical flexible sheet material in the formation of the bellows.